Isocyanate mixtures having CD and/or UI groups can be prepared in a simple manner using the highly active catalysts of the phospholine series, in particular the phospholine oxide series, by the processes as described in U.S. Pat. Nos. 2,853,473, 6,120,699 and EP-A-0 515 933.
The high catalytic activity of the phospholine catalysts, in particular the phospholine oxide catalysts, is desirable on one hand in order to promote the carbodiimidization reaction under mild temperature conditions, but on the other hand the effective stopping of the phospholine catalysis or the phospholine oxide catalysis has up to now not been without challenges. The carbodiimidized isocyanates tend to undergo an after-reaction, i.e. they outgas as a result of CO2 evolution. This then leads, particularly at relatively high temperatures, to a buildup of pressure, for example in the storage vessels. In addition, obtaining light-colored products and in particular the color stability after prolonged storage can be problematic time and again.
There have been many attempts to find an effective way of stopping the phospholine catalysis. Such stoppers are, for example, mentioned in the patent applications DE-A-25 37 685, EP-A-0 515 933, EP-A-0 609 698, EP-A-1 607 425, U.S. Pat. No. 6,120,699 and US 2007/213496 and comprise, for example, acids, acid chlorides, chloroformates, silylated acids and halides of the main group elements. Stoppers disclosed in the prior art are, in particular, ethyl trifluoromethylsulfonate (ETF), trimethylsilyl trifluoromethanesulfonate (TMST) as methylating or silylating reagent or strong acids such as trifluoromethanesulfonic acid (TFMSA) or perchloric acid.
According to the teaching of EP-A-0 515 933, CD/UI-containing isocyanate mixtures prepared by means of phospholine catalysis are stopped using at least the equimolar amount, preferably the 1- to 2-fold molar amount, based on the catalyst used, of a silylated acid (or ester thereof), for example trimethylsilyl trifluormethanesulfonate (TMST). However, it has been found in practice that CD/UI-containing isocyanates prepared in this way have only limited suitability for the preparation of prepolymers, i.e. reaction products of these CD/UI-containing isocyanates with polyols. The reaction products prepared in this way from polyols and the CD/UI-modified isocyanates tend to outgas, which can lead to a buildup of pressure in the transport containers or to foaming when handling such products.
This problem can be circumvented by using the silylated acid employed for stopping the phospholine catalyst in a manner analogous to EP-A-0 515 933 in higher molar equivalents (e.g. from 5:1 to 10:1, based on the catalyst). However, it is found in practice that the CD/UI-modified isocyanates obtained then have a significantly poorer color number. This then also applies to the prepolymers prepared therefrom.
This also applies when the phospholine catalyst is stopped using acids of the trifluoromethanesulfonic acid type as per U.S. Pat. No. 6,120,699. Prepolymers prepared therefrom also have a considerably increased color number.
According to the teaching of EP-A-1 616 858, an unsilylated acid and/or an acid chloride and/or sulfonic ester is used in addition to the silylated acid described in EP-A-0 515 933. However, this is also not able to solve the indicated problems completely since the catalyst can become less active when subjected to temperature stress during storage or during further processing to give (semi)prepolymers.
According to the teaching of EP-A-1 671 988, an alkylating agent, preferably an ester of trifluoromethanesulfonic acid, is used as stopper. However, this method has the disadvantage that the condensation reaction can start again on addition of polyols as are used in the preparation of prepolymers, which leads to lower than expected isocyanate contents.
WO 2007/076998 teaches carrying out the carbodiimidization in the presence of a secondary or tertiary amine and stopping the reaction by means of the known reagents of the prior art. These additives serve to bind the hydrochloric acid which is present in traces in the isocyanate as a result of the method of production and impairs carbodiimide formation, and only indirectly increase the stability of the product because the catalyst concentration can be reduced only slightly without increasing the reaction times.
US 2007/213496 discloses the preparation of carbodiimide-containing polyisocyanates in the presence of phospholine oxide derivatives as catalysts at moderate reaction temperatures of from 80° C. to 130° C. Stopping of the reaction is carried out in two stages with gradually decreasing temperature. In the first stage, an acid, a peroxide or acid chloride is preferably used as stopper.
It is common to all the processes described in the prior art that the stability of the resulting carbodiimide-containing polyisocyanates during prolonged storage and/or under temperature stress and/or in the case of a pH change (the latter two aspects are particularly important for further processing to form prepolymers) is frequently unsatisfactory. This can result in worsening of color and renewed commencement of the condensation reaction to the carbodiimide which proceeds with elimination of CO2. Even without prolonged storage, the color of carbodiimide-containing polyisocyanates is not always satisfactory.
Further improvements in this field of technology would therefore be desirable. In particular, it would be desirable to have available a process for preparing liquid isocyanate mixtures having carbodiimide and/or uretonimine groups, which does not have the abovementioned defects. The liquid isocyanate mixtures having carbodiimide and/or uretonimine groups to be prepared should, in particular, have low color numbers and be storage-stable, i.e. the color value, NCO content and viscosity should change very little during storage. In particular, it would also be desirable for the liquid isocyanate mixtures having carbodiimide and/or uretonimine groups prepared in this way to be able to be converted into prepolymers having a low color number and high storage stability.